So, here's my first proposal for new forum icons based on speech bubbles in stead of envelopes. What am I trying to achieve here is:
- first, a nice generic speech bubble of course
- then adding variations to indicate the different states (new, hot, sticky, closed) in a way that allows for combining them (hot & new)
- I've refrained from using colors in these first attempts to force myself to come up with visually distinct icons based on shape alone.

Some things to consider when giving feedback:
- General look and feel, I'm trying to keep it simple and effective, hopefully not boring?
- Is there enough difference between them?
- Metaphors, do they make sense to you?
- File format: 24bit or 8 bit pngs, transparency issues in IE…

I'm working on a mockup that'll show this icons in context, based on a screengrab from the drupal.org forums.

So as I said in IRC, I'm really enthusiastic about these. No envelopes and flames is a winner in my book. The icons seem clear and recongnisable. The hot topic icon is a very clever idea. The designs are friendly looking without being distracting or too stylised towards one kind of site.

The large dot for unread posts is visually clear, although I'm not sure how understandable it is without explanation. Might be a good idea to show the set to people without the labels to see if they understand what they mean. To be fair, they should be shown in context on a forum mockup. So I'll wait until that mockup is done.

Also, I'd like to see these in context to see how scannable they are. I think we will probably find we need colour to really make them scannable, but the icons are so clear that I think colour is an enhancement rather than a solution to a problem.

Regarding format: I guess if we don't have alpha transparency we will have to put a box around these to make them look good in all situations. That would be a bit unfortunate, however. I think most examples I've seen get round this problem by having rather square designs with no antialiasing and they look ugly for it. The default fallback background colour for the transparent sections in IE6 is grey, but this can be changed to whatever we want. So I guess part of this depends on whether we are happy to have IE6 users see a coloured square as a background to the image. I think I am generally in favour of doing that.

Thanks for all your feedback. Attached a mockup of the icons "in action". You'll see the big dot for "new" makes them quite scannable.
Color would definately enhance this, but I think they already work as is.

I chose the dot because that's how unread messages are marked in my mail app (OS X Mail). Is that a common thing in your inboxes too?

Bloomaniac, Brightloudnoise:
Sorry about duplicating http://drupal.org/node/102743, should have searched. I'll attach a layered .tif file (remove the .txt extension to expand the zip) with the forum screengrab as well and I ask you to demo your proposals in a 16x16pixel real-use case here for a good comparison, ok?